1. Field of the Invention
The present disclosure relates to a method and apparatus for acquiring orthogonal frequency division multiplexing (OFDM) synchronization, and more particularly, to a method and apparatus for acquiring OFDM synchronization by performing timing synchronization and at the same time reexamination of packet detection even when it fails to detect a plurality of symbols at an initial stage.
This work was supported by the IT R&D program of MIC/IITA [Work Management Number: 2006-S-071-02, Work Name: the Development of Super-high Speed Multimedia Transmission Solution]
2. Description of the Related Art
A transmitter of an MB-OFDM UWB system transmits a preamble packet for synchronization before transmitting data, where MB-OFDM UWB stands for multi band orthogonal frequency division multiplexing ultra wide band.
A receiver of an MB-OFDM UWB system performs packet detection, timing synchronization, frequency offset estimation, and frame detection using correlation between a received preamble and a preset preamble.
FIG. 1 is a diagram illustrating a structure of an OFDM preamble according to the related art.
As shown in FIG. 1, the OFDM preamble includes 24 time domain data 140 for packet detection, timing synchronization, frequency offset estimation, and frame synchronization, and 6 frequency domain data 150 for channel estimation. That is, the OFDM preamble includes total 30 OFDM symbols 140 and 150.
A transmitter alternatively transmits 30 OFDM preambles 140 and 150 using three different frequency bands 110, 120, and 130, each of which has an individual bandwidth of 528 MHz.
A receiver performs packet detection and timing synchronization acquisition through correlation between the received OFDM preambles 140 and 150 and a preset OFDM preamble.
Hereinafter, the three difference frequency bands 110, 120, and 130 are referred as a first frequency band 110, a second frequency band 120, and a third frequency band 130 for convenience, respectively.
FIG. 2 is a diagram illustrating a structure of an individual OFDM symbol according to the related art. That is, each of 24 OFDM symbols has a structure shown in FIG. 2.
As shown in FIG. 2, the individual OFDM symbol includes an N-point OFDM symbol 210 formed of preamble pattern data for synchronization acquisition, and an M-point zero value 220 used for a guard interval.
The N-point OFDM symbol 210 is a preamble pattern, which is set according to related standard and known to a transmitter and a receiver.
Therefore, the receiver can perform packet detection by calculating a cross correlation value between a received preamble and a preset preamble.
The M-point zero value 220 is a guard interval between valid data. That is, the M-point zero value 220 is invalid data for protecting valid data.
FIG. 3 is a diagram illustrating detection of OFDM preamble and cross correlation value according to the related art. With reference to FIG. 3, operations of a receiver will be described. That is, the receiver performs packet detection, timing synchronization, frequency offset estimation, and frame detection of a receiver by calculating a cross correlation value between a preset preamble and a preamble received through three different frequencies, which described above.
At step S310, the receiver detects packets by adjusting a frequency to a first frequency band (band #1) 110 and receiving a preamble symbol of the first frequency band 110.
In detail, the receiver receives a first preamble symbol and a fourth preamble symbol, which are transmitted through the first frequency band 110, and determines that a cross correlation value 350 thereof is larger than a predetermined reference, thereby detecting a packet.
Here, the receiver cannot receive a second preamble symbol and a third preamble symbol which are transmitted through a second frequency band (band #2) 120 and a third frequency band (band #3) 130 because frequency hopping is not performed.
As described above, the receiver reconfirms the cross correlation value 350 for performing packet detection in order to prevent error from being generated by noise and interference.
At step S320, the receiver acquires timing synchronization using a cross correlation value between a sixth preamble symbol and a ninth preamble symbol.
Then, the receiver estimate frequency offset using the 14th preamble symbol to the 21st preamble symbol at step S330.
At step S340, the receiver performs frame detection using the 19th preamble symbol to the 21st preamble symbol.
Here, frame detection is a process of detecting a starting point of a data symbol by detecting the end of a preamble symbol duration.
As described above, the receiver perform packet detection, timing synchronization, frequency offset estimation, and frame detection through calculating a cross correlation value for 21 continuous preambles among total 24 preambles based on a method for acquiring OFDM synchronization according to the related art.
Therefore, when the receiver fails to detect the first preamble symbol while performing synchronization based on the method for acquiring OFDM synchronization according to the related art, the receiver cannot perform packet detection, timing synchronization, frequency offset estimation, and frame detection.